Spectroscopic Observations of Lyman-Break Galaxies at Redshift ~ 4, 5 and 6 in the GOODS-South Field

1.4 MB·English

by  E. Vanzella

Spectroscopic Observations of Lyman–Break Galaxies
at Redshift ∼ 4, 5 and 6 in the GOODS–South Field1
E. Vanzella2, M. Giavalisco3, M. Dickinson4, S. Cristiani2,10, M. Nonino2,
H. Kuntschner5, P. Popesso6, P. Rosati6, A. Renzini7, D. Stern8,
9
0 C. Cesarsky6, H. C. Ferguson9, R.A.E. Fosbury5,
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and the GOODS Team
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a 2INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 40131 Trieste, Italy
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7 3Astronomy Department, University of Massachusetts, Amherst MA 01003, USA
2

4NOAO, PO Box 26732, Tucson, AZ 85726, USA
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C 5ST–EFC, Karl Schwarzschild Strasse 2, 85748, Garching, Germany
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h
6ESO, Karl Schwarzschild Strasse 2, 85748, Garching, Germany
p
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o
7INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova,
r
t
s Italy
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[
8JPL, California Institute of Technology, Mail Stop 169-527, Pasadena, CA 91109

1
v
9STScI, 3700 San Martin Dr., Baltimore, MD 21218, USA
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6
3 10 INFN, National Institute of Nuclear Physics, via Valerio 2, I-34127 Trieste, ITALY
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1
0 ABSTRACT
9
0
:
v We report on observations of Lyman–break galaxies (LBGs) selected from
i
X
the Great Observatories Origins Deep Survey (GOODS) at mean redshifts z ∼ 4,
r
a 5 and 6 (B435–, V606– and i775–band dropouts, respectively), obtained with the
red–sensitive FORS2 spectrograph at the ESO VLT. This program has yielded
spectroscopic identifications for 114 galaxies (∼ 60% of the targeted sample), of
which 51 are at z ∼ 4, 31 at z ∼ 5, and 32 at z ∼ 6. We demonstrate that
1
BasedonobservationsmadeattheEuropeanSouthernObservatoryVeryLargeTelescope,Paranal,Chile
(ESO programme 170.A-0788 The Great Observatories Origins Deep Survey: ESO Public Observations of
the SST Legacy / HST Treasury / Chandra Deep Field South). Also based on observations obtained with
theNASA/ESAHubble Space TelescopeobtainedattheSpaceTelescopeScienceInstitute,whichisoperated
by the Association of Universities for Research in Astronomy, Inc. (AURA) under NASA contract NAS
5-26555.
– 2 –
the adopted selection criteria are effective, identifying galaxies at the expected
redshift with minimal foreground contamination. Of the 10% interlopers, 83%
turn out to be Galactic stars. Once selection effects are properly accounted for,
the rest–frame UV spectra of the higher–redshift LBGs appear to be similar
to their counterparts at z ∼ 3. As at z ∼ 3, LBGs at z ∼ 4 and z ∼ 5
are observed with Lyα both in emission and in absorption; when in absorption,
strong interstellar lines are also observed in the spectra. The stacked spectra of
Lyα absorbers and emitters also show that the former have redder UV spectra
and stronger but narrower interstellar lines, a fact also observed at z ∼ 2 and
3. At z ∼ 6, sensitivity issues bias our sample towards galaxies with Lyα in
emission; nevertheless, these spectra appear to be similar to their lower–redshift
counterparts. As in other studies at similar redshifts, we find clear evidence
that brighter LBGs tend to have weaker Lyα emission lines. At fixed rest–
frame UV luminosity, the equivalent width of the Lyα emission line is larger at
higher redshifts. At all redshifts where the measurements can be reliably made,
the redshift of the Lyα emission line turns out to be larger than that of the
interstellar absorption lines, with a median velocity difference ∆V ∼ 400 km s−1
at z ∼ 4 and 5, consistent with results at lower redshifts. This shows that
powerful, large–scale winds are common at high redshift. In general, there is no
strong correlation between the morphology of the UV light and the spectroscopic
properties. However, galaxies with deep interstellar absorption lines and strong
Lyα absorption appear to be more diffuse than galaxies with Lyα in emission.
Subject headings: cosmology: observations — galaxies: formation — galaxies:
evolution — galaxies: distances and redshifts
1. Introduction
The study of galaxies at high redshift is crucial for understanding the formation of the
Hubblesequence, thegrowthofvisible structures intheUniverse andtheprocesses leadingto
thereionizationoftheintergalactichydrogenattheendoftheDarkAges. Inthepastdecade,
theempiricalinvestigation ofgalaxiesathighredshifts (i.e., z > 1.5)hasmaderapidprogress
thanks to advances in telescopes and instrumentation and to the development of optimized
selection techniques based on the observed colors of galaxies through either broad or narrow
passbands. Color–selection criteria, which are designed to target galaxies with a range of
spectral energy distributions (SEDs) within a targeted redshift window, are generally very
efficientandallowonetobuildlargesampleswithreasonablywellcontrolledsystematics(e.g.,
– 3 –
Steidel et al. 1999; Daddi et al. 2004; van Dokkum et al. 2003; Taniguchi et al. 2005),
suitable for a broad range of studies, both statistical in character or based on the properties
of the individual sources.
Among the various types of galaxies at high redshifts identified by color selection, the
Lyman–breakgalaxies(LBGs; e.g.,Guhathakurta et al. 1990;Steidel et al. 2003;Giavalisco et al.
2004b, forareview, seeGiavalisco2002)arethebest studiedandtheirsamplesarethelargest
both from a statistical point of view, and in terms of the the cosmic time covered (reaching
back to less than one billion years after the Big Bang). The reason is mostly practical: since
these galaxies are selected on the basis of luminous rest–frame ultraviolet (UV) emission,
which at redshifts 2.5 ∼< z ∼< 6 is redshifted into the optical and near–infrared windows, the
observations are among the easiest to carry out, taking advantage of very sensitive instru-
mentation with large areal coverage.
LBGsatredshift z ∼ 3havebeenintensively studiedwithbothverylarge(Steidel et al.
2003) and deep samples (Papovich et al. 2001), including high–quality spectra for more
than a thousand galaxies. Current surveys at z ∼ 3 provide the largest data set to study
the properties of galaxies during a relatively early phase of galaxy evolution (z ∼ 3 corre-
sponds to when the Universe was ∼ 20% of its current age), at least for one spectral type,
namely star–forming galaxies with moderate dust obscuration. A number of follow–up stud-
ies have been carried out following the discovery of these galaxies (Steidel et al. 1996a,b),
including studies of their morphology and size (Giavalisco et al. 1996; Papovich et al. 2003;
Ravindranath et al. 2006; Law et al. 2007; Lotz et al. 2006; Ferguson et al. 2004), their
ages and stellar masses (Papovich et al. 2000; Shapley et al. 2000; Dickinson et al. 2003a),
their chemical evolution (Pettini et al. 2000; Shapley et al. 2003), and of their clustering
properties (Giavalisco et al. 1998; Adelberger et al. 1998; Giavalisco & Dickinson 2001;
Adelberger et al. 2003).
At higher redshifts, LBGs appear fainter, the observations require higher sensitivity and
the samples are still relatively small. As a consequence, the properties of the most–distant
LBGs are less well characterized. Initial studies of the spectral properties and luminosity
function have been carried out at z ∼ 4 (Steidel et al. 1999) and z ∼ 5 (Madau et al.
1998) based on fairly small samples. More recently, larger samples at z > 4 have been gath-
ered, frombothground–andspace–basedobservatories(Shimasaku et al. 2005;Iwata et al.
2003; Giavalisco et al. 2004b; Dickinson et al. 2004; Bunker et al. 2004; Bouwens et al.
2006, 2007); these samples, however, are exclusively photometric ones, with small numbers
of spectroscopic identifications. Such studies have investigated a wide spectrum of statistical
propertiesofLBGsatz > 4anduptoz ∼ 7,suchasspatialclustering, morphology,UVlumi-
nosity function, stellar mass and properties of the Lyα line (see, for example, Hamana et al.
– 4 –
2006; Lee et al. 2006; Ravindranath et al. 2006; Lotz et al. 2006; Giavalisco et al. 2004b;
Ferguson et al. 2004;Ouchi et al. 2005;Bouwens et al. 2007;Ando et al. 2006,2007;Yan et al.
2006). However, these results are based on the assumption that the spectral properties of
LBGs at z ∼> 4 are the same as at z ∼ 3. It is reasonable to expect that the higher–redshift
samples should bear a similarity to those at z ∼ 3, since LBG color selection at all redshifts
are tuned to select galaxies with a similar rest-frame UV SED. However, without spectro-
scopic information, it is impossible to know if evolutionary effects are introducing systematic
biases in the observed statistics. For example, if the distribution of surface brightness, UV
SED or Lyα emission line properties evolve with redshift, this will affect the redshift distri-
bution and the completeness of the samples, which in turn will bias derived properties such
as the LBG spatial clustering, luminosity function, and the evolution of these quantities.1
In this paper we present results from a program of spectroscopic follow–up of LBGs
at redshift z > 4 selected from optical images obtained with the Advanced Camera for
Surveys (ACS; Ford et al. 1993) on board the Hubble Space Telescope (HST) in the four
passbands, B , V , i and z , as part of the Great Observatories Origins Deep Survey,
435 606 775 850
orGOODS(foranoverview oftheGOODSproject, seeRenzini et al. 2002;Dickinson et al.
2003b; Giavalisco et al. 2004a). The spectra have been obtained at the ESO VLT with the
FORS2 spectrograph. The data from this program, specifically all the spectra of galaxies
in the redshift range 0.5-6.3, have already been released and described in previous papers
(Vanzella et al. 2005, 2006, 2008). Here we focus on a sample of LBGs, which has yielded
114 spectroscopic identifications in the redshift interval 3.1 − 6.3. Re-analyzing the whole
LBG sample, we introduce very few differences (mainly in the quality redshift) to respect
the previous global release (Vanzella et al. 2008), improvements that have been marked in
the reported list of the present work. Currently, it represents one of the largest and most
homogeneously–selected spectroscopic samples in this redshift range.
Throughout thispaper magnitudesareintheABscale(Oke1974),andtheworldmodel,
when needed, is a flat universe with density parameters Ω = 0.27, Ω = 0.73 and Hubble
m Λ
constant H = 73 km s−1 Mpc−1.
0
1Note that a simple analysis of the observed colors or sizes is not sufficient to establish the presence of
evolutionaryeffects sinceit isnotpossibleto separatelymeasurethe distributionfunctions ofcolor,sizeand
luminosity. See discussion in Reddy et al. (2008).
– 5 –
2. Data and Sample Selection
2.1. ACS Images and Source Catalogs
We have selected samples of LBGs at mean redshift z ∼ 4, 5 and 6 (in the following
referred to as B –, V –, and i –band dropouts, respectively) from the latest version
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(v2.0) of the GOODS images, obtained with the ACS on HST. The v2.0 mosaics are nearly
identical in shape and size to the v1.0 ones. They cover the two GOODS fields, the northern
oneencompassing theHubbleDeepFieldNorth(HDF–N)andthesouthernonelocatedatthe
center of the Chandra Deep Field South (CDF–S). Each subtend an area of approximately
10×17arcminonthesky, foratotalarealcoverageofabout0.1squaredegrees. Aswithv1.0,
the v2.0 images consist of two sets of mosaics observed in the B , V , i and z filters.
435 606 775 850
The depth of the V , i and z mosaics, however, has been increased over version v1.0
606 775 850
by including additional observations taken during the continuation of the original GOODS
survey for high-redshift Type Ia supernovae (Riess et al. 2004, 2005). Since these additional
data were obtained using the same observational strategy as the original ACS program (e.g.,
the same Phase-II files were used to carry out the observations), integrating them into the
existing mosaics has been straightforward and has resulted in doubling the original exposure
time in the z band as well as a more modest depth increase in the other bands.2
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2.2. Photometric Samples of Lyman–Break Galaxies
We have selected samples of LBGs using color criteria very similar to those presented
by Giavalisco et al. (2004b; G04b hereafter), with some minor modifications applied to the
definition of B –band dropouts to explore the redshift distribution of galaxies near the
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border of that color–color selection window. The exact locations of such windows balance
the competing desires of completeness and reliability. Windows are designed to include
as complete a sample of target galaxies as possible given the dispersion of observed colors
— due both to both observational scatter and the intrinsic dispersion in galaxy UV SEDs
(e.g., related to varying dust content, ages, metallicities, Lyα equivalent widths, etc.). On
the other hand, windows are designed to avoid significant numbers of galaxies at redshifts
outside (usually lower than) the targeted one.
2Thev2.0exposuretimesintheB435 ,V606 ,i775 andz850 bandsare7200,5450,7028and18232seconds,
respectively. Detailsofthe ACSobservations,aswellasmajorfeaturesoftheGOODSproject,canbefound
inGiavalisco et al. (2004a);additionalinformationaboutthelatestv2.0releaseoftheGOODSACSimages
and source catalogs can be found at www.stsci.edu/science/goods/v2.0and will be described in detail
in an upcoming paper (Giavalisco et al., in prep.).
– 6 –
In the present work, B –band dropouts are defined as objects that satisfy the color
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equations:
(B −V ) ≥ 1.1+(V −z ) ∧ (B −V ) ≥ 1.1 ∧ (V −z ) ≤ 1.6, (1.1)
450 606 606 850 450 606 606 850
where ∧ and ∨ are the logical AND and OR operators. These criteria extend the selection
of candidates to slightly bluer (B -V ) and redder (V -z ) colors than those in G04b.
435 606 606 850
As can be seen in Figure 1, which shows the selection windows corresponding to both sets of
color equations, the sample selected with the new criteria (solid line) fully includes the one
selected with the G04b criteria (dashed line). We have decided to use these more general
criteriatodefinethesampleofB –banddropouts,whichisthelargestamongthethreeLBG
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samples targeted for the spectroscopic observations, to explore both changes in the low–end
of the targeted redshift range and contamination rates from low–redshift interlopers.
The definitions of the color equations of V –band and i –band dropouts are un-
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changed from those used in G04b and Dickinson et al. (2004), and are given by the color
equations
[(V −i ) > 1.5+0.9×(i −z )] ∨
606 775 775 850
∨ [(V −i ) > 2.0] ∧ (V −i ) ≥ 1.2 ∧ (i −z ) ≤ 1.3
606 775 606 775 775 850
∧ [(S/N) < 2] (1.2)
B
and
(i −z ) > 1.3 ∧ [(S/N) < 2] ∨ [(S/N) < 2], (1.3)
775 850 B V
respectively (see Figure 2 for a collapsed representation of the selection windows for V –
606
and i -band dropouts). For all three selection criteria above, when the isophotal S/N in a
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given band is less than one, limits on the colors have been calculated using the 1σ error on
the isophotal magnitude.
We have restricted the photometric samples to galaxies with isophotal S/N ≥ 5 in
the z band, and we have visually inspected each candidate, removing sources that were
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deemed artifacts. In addition, we have estimated the number of spurious detections using
counts of negative sources detected in the same data set. Together, these amount to a
negligible number of spurious sources for the B – and V –dropout samples, and ≈ 12%
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for the i –dropouts. We have also eliminated all sources with stellar morphology down
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to apparent magnitude z ∼ 26, i.e., where such a morphological classification is reliable.
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This accounts for an additional 3.1%, 8.3% and 4.6% of the B –, V – and i –dropout
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samples, respectively. While this procedure biases our samples against LBGs (and high–
redshift quasars) that are unresolved by ACS, it minimizes contamination from Galactic
stars. Note that we have spectroscopically observed a few point-like sources that obey the
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dropout selections in order to verify that such sources are indeed Galactic. In practice, these
cullings of the dropout samples result in negligible changes to the spectroscopic samples and
to key measured quantities, such as the specific luminosity density.
Down to z ≤ 26.5, roughly the 50% completeness limit for unresolved sources, the
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culled samples include 1544, 490 and 213 B –, V –and i –band dropouts, respectively.
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With a survey area of 316 arcmin2, this corresponds to surface density Σ = 4.89 ± 0.12,
1.55±0.07 and 0.67±0.05 galaxies per arcmin2 for the three types of dropouts, respectively.
Error bars simply reflect Poisson fluctuations.
We note that while the V – and i -band dropout samples are mutually exclusive (i.e.,
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i −z ≤ 1.3 vs. i −z > 1.3, respectively), the intersection of the B – and V -
775 850 775 850 435 606
band dropout samples may be non-zero. However, in this latter case, no sources in common
have been found down to the z ≤ 26.5, and only one galaxy satisfies both criteria when
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the magnitude limit is extended down to z ≤ 27.5 (i.e., GDS J033245.88-274326.3).
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We have used Monte Carlo simulations to estimate the redshift distribution function of
our LBG samples and compared the results to observations. The technique is the same as
that used in G04b and consists of generating artificial LBGs distributed over a large redshift
range (we used 2.5 ≤ z ≤ 8) with assumed distribution functions for UV luminosity (we used
a flat distribution, discussed below), SED, morphology and size. We adjusted the input SED
and size distribution functions by requiring that the distribution functions recovered from
the simulations match the z ∼ 4 observed sample, the largest of the three GOODS samples.
In this way, both simulations and observations are subject to similar incompleteness, photo-
metric errors (in flux and color), blending, and other measurement errors. The model SED
used for the simulations is based on a synthetic spectrum of a continuously star–forming
galaxy with age 108 yr, Salpeter IMF and solar metallicity (Bruzual & Charlot 2000). We
reddened it with the starburst extinction law (Calzetti 2000) and E(B − V) randomly ex-
tracted from a Gaussian distribution with µ = 0.15 and σ = 0.15. In other
E(B−V) E(B−V)
words, the dispersion of the LBG UV SEDs is modeled as only due to the dispersion in
the amount of obscuration for the same unobscured SED, neglecting the effects of age and
metallicity of the stellar populations. This is obviously a crude approximation, but, thanks
to the strong degeneracy between age, obscuration and metallicity on the broad–band UV
colors of star–forming galaxies, it is adequate here since we are only interested in measuring
the selection effects due to the specifics of the observations. For the cosmic opacity, we have
adopted the Madau (1995) prescription, extrapolated to higher redshifts when necessary. To
model the dispersion of the morphologies of the galaxies we have used an equal number of
r1/4 and exponential profiles with random orientation, and size extracted from a log–normal
distributionfunction(seeFerguson et al. 2004). Wefoundtheaverageredshift andstandard
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deviation of the redshift distribution to be z = 3.78 and σ = 0.34 for the B –dropout
B B 435
sample, z = 4.92 and σ = 0.33 for the V –dropout sample, and z = 5.74 and σ = 0.36
V V 606 i i
for the i –dropout sample.
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2.3. The Spectroscopic Sample
We have selected a sample of 202 LBGsfromthe three samples defined above asprimary
targets of the FORS2 spectroscopic observations. While the criteria to include a galaxy in
the target list were mostly based on its apparent magnitude, as we detail below, we did
not set a strict flux limit for the spectroscopic sample in these initial high-redshift LBG
spectroscopic studies. This allowed us to empirically assess how often the presence of Lyα
emission allows the measurement of the redshift of galaxies which are too faint for absorption
spectroscopy.
Targets were assigned slits in the FORS2 multi–object spectroscopic masks according to
an algorithm in which two competing factors combine to maximize (i) the number of targets
and (ii) the likelihood of success, under the assumption that brighter targets are more likely
to result in successful identifications. In practice, while brighter galaxies were more likely to
be assigned a slit, (slightly) fainter targets could still win the competition if their coordinates
allowed a larger total number of targets on a given mask. Relatively faint targets in close
proximity to brighter one were also assigned a slit if their inclusion could be made without
penalty. Where possible, we assigned faint targets to multiple masks.
When the number of available slits in a mask exceeded that of available targets, we
populated the remaining slits with “filler” targets selected to test target selection criteria
and to identify lower–redshift galaxies in the range z ∼ 1−2 (for a summary of the global
target selection of the FORS2 campaign, see Vanzella et al. 2006). In particular, we se-
lected some filler targets using LBG criteria that extended the primary B – and V –band
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dropout selection criteria to galaxies with less pronounced “Lyman drops” and bluer UV
continuum, thus getting closer to the locus of general field galaxies. As discussed earlier,
such observations are useful for exploring the dependence of the redshift distribution func-
tion of the confirmed LBGs on the details of the color selection, as well as for measuring
contamination by low–redshift interlopers. In what follows we refer to these more loosely
defined B – and V –band dropouts simply as “fillers”. Only three such spectroscopically-
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identified fillers are considered below, two B –dropout fillers (GDS J033234.40-274124.3at
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z = 3.418, QF=BandGDSJ033251.81-275236.5atz = 3.468, QF=A)andone V –dropout
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filler (GDS J033239.82-275258.1 at z = 5.543, QF=C). We will report more extensively on
these tests in following papers, which will also include spectroscopic observations of GOODS
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galaxies obtained with different instrumental configurations.
3. FORS2 Spectroscopic Observations
The details of the observations, including journals of the observing runs, data reduction,
the extractions of the spectra have been reported in Vanzella et al. (2005, 2006, 2008), and
we refer the reader to those papers. We recall that the wavelength coverage was typically
5700˚A-10000˚A with a spectral resolution of R = λ/∆λ = 660, corresponding to 13˚A at
8600˚A. No order separation filter was used.
In the vast majority of cases, the redshift has been calculated through the identification
of prominent features of LBG spectra, e.g., Lyα either in emission or absorption, and Siii
1260˚A, Oi+Siii 1302˚A (a blend at the spectral resolution of our instrumental setup), Cii
1335˚A, Siiv 1394,1403˚A, Siii 1527˚A, Civ 1548, 1551˚A in absorption.
Redshift determinations have been made based on visual identification of spectral fea-
turesaswellasbycross–correlatingtheobservedspectraagainsthigh–fidelityLBGtemplates
of differing spectral types using the rvsao package in the IRAF environment. In particular,
we used emission and absorption line LBG templates from Shapley et al. (2003) as well as
the lensed absorption line LBG cB58 (Pettini et al. 2000). Each two-dimensional spectrum
has been visually inspected, including consideration of its slit orientation on the sky. In
many cases where no continuum has been detected, we derive a redshift measurement from
Lyα emission.
We have co–added all repeated spectra to improve the final S/N. The typical exposure
time for each mask was about 14,400 seconds and for co-added sources, total exposure times
range from 20,000 to 80,000 sec (e.g., see Vanzella et al. 2008).
We have assigned each measured redshift a quality flag (QF), with values of either
A (unambiguous identification), B (likely identification; e.g., based on only one line or a
continuum break), or C (uncertain identification). The presence of Lyα emission in the
second order spectrum (at > 10,000˚A), has also been used on occasion, especially for faint
sources with low QFs based on absorption features. For example, one B –band dropout
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source(GDSJ033221.05-274820.5)showsanapparentlyfeaturelesscontinuumwithasecond-
orderemissionlineat∼10400˚A,implyingz ∼3.3. Indeed, recentlytheVIMOSspectroscopic
observations have confirmed this galaxy to be at z = 3.385 (Popesso et al. 2008).
We have assigned a redshift to 118 galaxies of the initial list of 202 targets, or 58.4%
of the input list; this relatively low success rate is, in large part, due to two factors: (i) the
– 10 –
target list includes a relatively large fraction of faint sources — 65 or 32.2% of the sample
have z > 26; and (ii) the difficulty in deriving redshifts for galaxies at z < 3.6 with our
850
instrumental configuration. In the latter case, depending on the slit position, Lyα and the
UV absorption features are often blueward of the spectral range available.
Of the 118 spectroscopically identified sources, 106 have redshifts in the expected range
for their adopted color selection. Note that some of these redshifts have already been pub-
lishedinVanzella et al. (2005,2006). Ofthesourcesoutsidetheexpectedredshift range, one
source is a low–redshift galaxy from the B –band dropout sample, one is a low–redshift
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galaxy from the V –band dropout sample, and 10 are Galactic stars (1, 3 and 6 from
606
the B –, V –, and i –dropout samples, respectively). We note that one faint star,
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GDS J033238.80-274953.7 (z = 25.16), that we spectroscopically classify with QF=C, has
850
been confirmed Galactic in nature due to the detection of its proper motion (M. Stiavelli,
private communication). Excluding the Galactic stars and the two low–redshift interlop-
ers, the final list of spectroscopically identified LBGs includes 46 B –band dropouts, 32
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V –band dropouts, and 28 i –band dropouts (reported in Tables 1, 2 and 3).
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Asmentionedabove,wehaveassignedredshiftstothreehigh–redshiftfillertargetsfound
to be in the same redshift range as the primary LBG sample. This brings the total number
of high-redshift (z > 3.1) spectroscopic identifications to 109, of which 32 have QF=C. Of
these 109 galaxies, 70 have redshift z > 4 (24 with QF=C); 37 have redshift z > 5 (13 have
QF=C); and 32 have redshift 5.5 < z < 6.5 (11 with QF=C; see Table 4 for a summary).
Finally, we also found five serendipitously–identified, high–redshift galaxies. These fell,
as second or third sources, on slitlets assigned to other primary targets. For four of them
the redshift identification relies upon a Lyα emission line; only in one case does the redshift
rely upon absorption features. These five serendipitous sources are, in right ascension order:
• GDS J033218.27-274712.0, at z = 4.783 (QF=C), marked in Figure 2 with a red
pentagon, is close to the V -band dropout selection window (V -i >1.901, i -
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z =0.501).
850
• GDS J033219.41-274728.4, at z = 3.250 (QF=C). This galaxy shows a flat continuum
and an absorption doublet interpreted as Civ 1548, 1551˚A.
• GDS 03322.89-274521.0, at z = 5.128 (QF=C). This source, clearly visible in the i
775
band (from which the coordinates were measured) is not detected in the B , V , or
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z bands. We assume that the emission line is Lyα, though lacking firm constraints
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on the continuum SED, we can not rule out another interpretation such as [Oii]3727
at redshift z = 0.999. This source is not used in the following analysis.